The refractive index of a medium is commonly measured by observing the phase change of an electromagnetic wave traversing a given distance within the medium.
Methods of measuring the refractive index field of a non-uniform specimen are well known. These methods rely on the transmission of a light wave through the medium in question. After it has traversed the medium, a light wave interferes with a reference wave which has not traversed the medium and the interference pattern so obtained is analyzed by means of a computer to give the refractive index in each pixel of the field. The refractive index has been interpreted in various ways, to indicate temperature or solute concentration, for example. The resolution of the methods used to date are limited by the accuracy to which the optical phase change can be determined at each point in the field. This limit is typically 2π/100 for a single pixel, and may be 2π/1000 for a complete field.
U.S. Pat. No. 5,120,131 (Lukosz), titled METHOD AND APPARATUS FOR SELECTING DETECTION OF CHANGES IN SAMPLES BY INTEGRATED OPTICAL INTERFERENCE, filed Aug. 10, 1990, is an example for such method. In an integrated optical interference method, polarized laser light is coupled into a planar waveguide, propagates in the waveguide as a guided wave, which consists of two mutually coherent, orthogonally polarized modes, interacts at least once with the sample, which is applied to the surface of a section of the waveguide called measuring section, and subsequently is coupled out of the waveguide. The time dependent phase difference between the two orthogonal polarization components of the outcoupled light is measured with a device comprising photodetectors and polarization optical components.
In U.S. Pat. No. 5,804,453 (Chen), filed Feb. 9, 1996, titled FIBER OPTIC DIRECT-SENSING BIOPROBE USING A PHASE TRACKING APPROACH there was disclosed a method of determining the concentration of a substance in a sample solution using a fiber optic probe having a reagent at its distal end to which the substance bonds. It comprises a fiber optic bioprobe in which an immobilized biolayer, such as an antigen or antibody, acts as a sensing etalon of a Fabry-Perot interferometer. The bioprobe is used in a system in which a shift in the spectral dispersion pattern, caused by reflected out-of-phase beams, is used to determine a concentration of a substance in a sample solution. The method disclosed in this patent investigates the phase difference in the sample caused by the presence of the substance in the sample solution, by examining the interference patterns of the sample. The method does not include the employment of surface plasmon resonance, nor does it provide an image of the sample.
In U.S. Pat. No. 5,442,448 (Knoll), filed Mar. 21, 1994, titled DEVICE FOR THE LATERALLY RESOLVED INVESTIGATION OF A LATERALLY HETEROGENEOUS ULTRATHIN OBJECT LAYER, there was disclosed a device for the investigation of a laterally heterogeneous ultrathin object layer, especially for the laterally resolved detection of a change in the layer thickness of the object layer which results from a specific binding reaction of a first binding partner bound to the object layer with an unbound second binding partner. The device has a multi-layer structure which extends into the investigation region and includes the object layer as first layer and a second layer adjacent to the latter, a coupler arrangement for the coupling of excitation light into the second layer. The excitation light generates at least one defined angle of incidence in the second layer a bound, non-radiating electromagnetic wave, from which an evanescent wave extends into the object layer. An optical imaging system is provided for the imaging of the investigation region of the object layer in an image plane at a defined angle of emergence, at which detection light is coupled out from the second layer. The coupler arrangement includes an optical grating structure which extends into the investigation region parallel to the object layer and which is rotatable about an axis extending perpendicular to the grating plane. This allows adjustment of the image contrast, by varying the orientation of the grating lines relative to the plane of incidence. In contrast to U.S. Pat. No. 5,804,453 (Chen) this invention does employ surface plasmon resonance on metal, and does provide an image, but it is not concerned with the phase change on reflection from the sample, nor does it employ interference.
In U.S. Pat. No. 5,313,264 (Ivarsson et al.), filed May 10, 1991, titled OPTICAL BIOSENSOR SYSTEM, there was disclosed an optical biosensor system using internal reflection versus angle of incidence determination for the detection of biomolecules, the system comprising a sensor unit with at least two sensing surfaces, a source of light, and means for forming a convergent beam of light which is focused in a wedge-shape fashion to form a streak of light extending transversely over all the sensing surfaces; a photodetector device; in the form of a two-dimensional matrix of individual photodetector; optical imaging instrumentation in the form of an anamorphic lens system for the purpose of imaging rays of reflected light from the sensing surfaces on each its own column of photodetectors, so that for each sensing surface there is a corresponding set of columns of photodetectors; and an evaluation unit for determining the minimum reflectance or the resonance angle at each of the sensing surfaces. The invention also relates to a method for calibrating the biosensor system, a method for correcting for baseline drift as well as a method for temperature regulation of thermostat means in the biosensor system. This system employs surface plasmon resonance, but does not investigates the phase nor does it provide an image, and is merely concerned with local sensing.
In U.S. Pat. No. 5,237,392 (Hickel et al.), filed Aug. 13, 1992, titled DETERMINATION OF REFRACTIVE INDEX AND THICKNESS OF THIN LAYERS, the refractive index and thickness of ultrathin layers less than 1 micrometer in thickness are determined by recording the layers, which have been applied to a solid support, by surface plasmon microscopy as a function of the angle of incidence of the incident laser beam, the method making it possible to determine layer thicknesses with a vertical resolution greater than or equal to 0.1 nm and a simultaneous lateral resolution greater than or equal to 5 μm. This invention deals with surface plasmon resonance and produces an image, but does not examine the phase change of the reflected or transmitted light.
Plasma resonance in a metal film has been used to increase the contrast of images of objects characterized by very small variations of optical refractive index (see Hickel W., Rothenhauser B., and Knoll W., Surface plasmon microscopic characterization of external surfaces, J. App. Phys. 66(1989) 4832–4836, Rothenhauser B., and Knoll W., Surface-plasmon microscopy, Nature 332 (1988) 615, and Yeatman E. and Ash E. A., Surface-plasmon microscopy, Elec. Lett 23 (1987) 1091).
It is a purpose of the present invention to provide a method of displaying the refractive index of each pixel of the image of a sample, using an accurate method of measuring the optical phase change at each point In the field of view, similarly to the method described Kostianovsky et al. (Kostianovsky S., Lipson S. G., and Ribak E., Appl. Optics 32 (1993) 4744, and see also Raz E. et al., Phys. Rev. A 40 (1989) 1089). In addition, it is a purpose of this invention to provide an experimental method and apparatus to carry out such a measurement and provide the said display of the refractive index field in the form of an image.